Extremely tired or exhausted. So weary that one can barely function.
This expression dates to the 1600s and likely comes from observing working dogs after a long day of hunting, herding, or other labor. Dogs would collapse and sleep deeply after intense physical work, providing a vivid image of complete exhaustion that resonated with people who worked equally hard.
The phrase captures the enviable ability of dogs to transition instantly from intense activity to deep rest, something humans often struggle with. It's one of many expressions that use animal behavior to describe human states, showing our long history of learning about ourselves by observing our animal companions.
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